Electromagnetic interference (EMI) is a generic term for unwanted interference energies either conducted as currents or radiated as electromagnetic fields. EMI can emanate from computers or other digital devices in several ways. Generally, voltages and currents in board mounted, integrated switching circuits create electric and magnetic fields that radiate from the device. EMI radiating from such devices will vary in field strength and impedance according to the shape and orientation of the conductors, the distance from the conductors to any shielding provided by circuit components or by coupling to circuit components.
Since most computers operate by generating pulse signals at rates in excess of a million pulses per second, radiating EMI will extend into the radio frequency spectrum and can cause significant interference with radio and television signals. The United States Federal Communications Commission has promulgated rules and procedures requiring the minimization and regulation of EMI by manufacturers. Under such regulations, EMI must be limited for certain devices to particular micro-volt ranges depending on the frequency of the EMI signal.
In lieu of the problems surrounding EMI and the need to comply with government regulations, schemes have been proposed for limiting EMI. One typical scheme has been to provide a conductive enclosure to an electronic device so that EMI field lines will terminate on such enclosure. Unfortunately, if multiple printed circuit boards are contained in the enclosure, EMI emanating from one board can effect the operation of another board within the enclosure.
In the past, designers have constructed multi-layer printed circuit boards in which the so-called power plane is positioned beneath the ground plane in an attempt to limit EMI. Unfortunately, such designs present greater construction complexities, since the leads of devices mounted on the circuit board designed for connection to the power plane and all conductive structures designed for power plane connection must pass through the ground plane.
Consequently, a need exists for controlling EMI emanating from a printed circuit board, whereby multiple printed circuit boards can be installed in a single electronic cabinet without inter-board electromagnetic interference.